In recent years, development of semiconductor devices in which a thin film transistor (TFT) and a thin film diode (TFD) are formed on the same substrate, and electronic devices including such a semiconductor device have been promoted. As for the method of manufacturing such a semiconductor device, there has been suggested a method of forming a semiconductor layer for a TFT and a TFD using the same crystalline semiconductor film formed over the substrate.
Patent Document 1 discloses an image sensor, which includes an optical sensor part using a TFD and a driver circuit using a TFT on a same substrate. In Patent Document 1, an amorphous semiconductor film formed on the substrate is crystallized to form the semiconductor layers for a TFT and a TFD.
When uniformly forming a TFT and a TFD on a same substrate in this manner, it is possible to make a semiconductor device smaller as well as to gain significant cost merits such as reduction in the number of components. Further, it also becomes possible to achieve a product including new functions, which cannot be obtained by combining conventional components.
Meanwhile, Patent Document 2 discloses using the same semiconductor film (amorphous silicon film) to form a TFT made of crystalline silicon (crystalline silicon TFT), and a TFD made of amorphous silicon (amorphous silicon TFD) on the same substrate. Specifically, a catalytic element for promoting crystallization of amorphous silicon is only added to a region where an active region of the TFT will be created in the amorphous silicon film formed on the substrate. A heating treatment is then performed, forming a silicon film in which only a region where an active region of a TFT will be created is crystallized, and a region that will be a TFD is in an amorphous state. It becomes possible to easily create a crystalline silicon TFT and an amorphous silicon TFD on the same substrate by using this silicon film.
Further, Patent Document 3 discloses a method of using the same semiconductor film (amorphous silicon film) to form crystalline semiconductor layers that have different crystal conditions as the active regions of a TFT and a TFD. Patent Document 3 suggests optimization of device characteristics required for the TFT and the TFD by making the crystallinity of the TFT semiconductor layer higher than the crystallinity of the TFD semiconductor layer.